Quote from TonyD on September 29, 2011, 15:08
If you want multiple serial ports then you can use the SPI or I2C interface to add a dual or quad SPI/I2C UART controller. The MAX14830 will do a quad UART with SPI or I2C interface. Providing a software driver for them will be the hard part
MAX14830 seems to be made of unobtainium, not even listed as out of stock by Digikey, Mouser, and Futurelec. They only seem to carry the single-uart versions, and those are $5 in hundred quantity. Much cheaper to just use a PIC or other microcontroler ($1-$2).